A locking device for securing an attachment member to a main body including first engagement pawls provided in the attachment member so that the first engagement pawls extend in the direction of attachment-detachment of the attachment member and along the inner surface of the main body and a push button assembly secured to the inner surface of the main body. The push button assembly includes a back panel for being secured to an inner surface of the main body, a push button for protruding through an opening in the main body, second engagement pawls formed on the push button and a compression spring provided between the push button and the back panel.